i62 THE POPULAR SCIENCE MONTHLY. 



tions of the geologic brevity of tlie time since the ice-sheets dis- 

 appeared from North America and Europe make it clear, in the 

 opinions even of some of the geologists who believe in a duality 

 or plurality of Quaternary Glacial epochs, that not astronomic 

 but geographic causes produced the Ice age. 



Glacialists who reject Croll's ingenious and brilliant theory 

 mostly appeal to great preglacial altitude of the land as the chief 

 cause of the ice accumulation, citing as proof of such altitude the 

 fiords and submarine valleys which on the shores of Scandinavia, 

 and the Atlantic, Arctic, and Pacific coasts of North America, 

 descend from one thousand to three thousand and even four thou- 

 sand feet below the sea level, testifying of former uplifts of these 

 continental areas so much above their present heights. But 

 beneath the enormous weights of their ice-sheets these lands sank, 

 so that when the ice attained its maximum area and thickness 

 and during its departure the areas on which it lay were depressed 

 somewhat lower than now and have since been re-elevated. This 

 view to account for the observed records of the Glacial period is 

 held by Dana, Le Conte, Wright, Jamieson, and others, including 

 the present writer. It is believed to be consistent either with the 

 doctrine of two or more glacial epochs during the Quaternary 

 era, or with the reference of all the glacial drift to a single glacial 

 epoch, which is thought by Wright, Prestwich, Lamplugh, Fal- 

 san, Hoist, Nikitin, and others to be more probable. To myself, 

 though formerly accepting two glacial epochs with a long warm 

 interval between them, the essential continuity of the Ice age 

 seems now the better provisional hypothesis, to be held with can- 

 dor for weighing evidence on either side. 



The duration of the Ice age, if there was only one epoch of 

 glaciation, with moderate temporary retreats and readvances of 

 the ice border sufficient to allow stratified beds with the remains 

 of animals and plants to be intercalated between accumulations 

 of till, may only have comprised a few tens of thousands of years. 

 On this point Prof. Prestwich has well written as follows : " For 

 the reasons before given, I think it possible that the Glacial epoch 

 that is to say, the epoch of extreme cold may not have lasted 

 longer than from fifteen thousand to twenty-five thousand years, 

 and I would for the same reasons limit the time of . . . the melt- 

 ing away of the ice-sheet to from eight thousand to ten thousand 

 years or less." 



From these and foregoing estimates, which seem to me accept- 

 able, we have the probable length of Glacial and postglacial time 

 together thirty thousand or forty thousand years, more or less ; but 

 an equal or considerably longer preceding time, while the areas 

 that became covered by ice were being uplifted to great altitudes, 

 may perhaps with good reason be also included in the Quaternary 



